Abstract This paper presents a model that predicts fiber–matrix separation in ribbed sections of compression molded parts. The model combines a mechanical analysis of compression molding with some experimentally measured variables. It is shown that with a higher closing speed, the viscosity of the material will increase and fiber–matrix separation can be reduced. Specific applications for this method are compression molding of sheet molding compounds and glass mat‐reinforced thermoplastics. POLYM. COMPOS., 28:451–457, 2007. © 2007 Society of Plastics Engineers